1. Field of the invention
The present invention relates to a low expansion cast alloy showing an average thermal expansion coefficient of not higher than 1.5.times.10.sup.-6 /.degree.C. at a temperature in the range of -50.degree. to 120.degree. C.
2. Description of the Prior Art
Recently, technology including a processing technique has been developed in respect of minuteness and thus there is an increasing need for effecting a size control in the order of microns or submicrons. In general, a substance shows an expansion or contraction phenomenon in accordance with the change of a temperature, and so, in order to have size control in the submicron order, it is required to meet either one or both of the requirements as follows:
(1) to keep the ambient environments at a constant temperature, and
(2) to use a material which retains low expansion in a wide temperature range.
As to the latter requirement (2), for example in iron base alloys, the so-called "invar" material of 36 Ni (nickel) series and also "super invar" material of 31 Ni-5 Co (cobalt) series are well known, and have average thermal expansion coefficients of .alpha..sub.0.degree. to 100.degree. C. .ltoreq.2.times.10.sup.-6 /.degree. C. and .alpha..sub.0.degree. to 100.degree. C. .ltoreq.1.times.10.sup.-6 /.degree. C., respectively. The "super invar" material, which has the lowest expansion coefficient among the commercially available iron based alloys, has been produced in a considerable amount, but it is provided as a forged or rolled material without exception. As the usage is broadened and the production amount accordingly is increased, however, problems have been noted in respect of both the use conditions and the productivity.
The stable structure of the "super invar" material comprises a single austenite phase, but its chemical component composition giving the low thermal expansion is close to the composition giving the martensitic transformation when it is cooled from a high temperature. Therefore, there is a great risk of martensite generation even by a slight fluctuation in the chemical composition.
Further, the martensitic transformation temperature (Ms point) considerably changes dependent upon the relative amounts of Ni and Co components. In effect, the Ms point normally is considered as not higher than -70.degree. C., but particularly in a composition of low Ni content, it is possible that the Ms point elevates nearly to the ice point or freezing-point (0.degree. C.). Accordingly, there is a problem in the special use at a temperature below the ice point. For example, in a high-altitude flight, the temperature of the luggage space of an aircraft is lowered nearly to -40.degree. C. For this case, the martensitic transformation occurs in the course of transportation, resulting in an extremely high thermal expansion coefficient. It is therefore required to strictly control the composition in a narrower range, even for a forged or rolled material.
On the other hand, in respect of productivity, the "super invar" material is not so hard but is a representative difficult to machine. As the demand has increased, machining processes required from the rough shape to the final product naturally have increased. To reduce the time of machining processes, trials have been made in order to obtain a work piece finished in the shape almost equal to the final product by a casting process such as an investment casting mold process. However, the cast products have greater micro-segregation than the forged or rolled products and so said trials have not yet been successful in providing a practicable process suitable for mass-production. In effect, even when a cast product is produced in compliance with the known requirements for chemical composition of the forged or rolled products, the properties of "super invar" material are often lost, because considerable segregation occurs particularly as to Ni, which is an austenite stabilizing element, and because the martensite is generated even at a normal temperature in the local Ni poor portions. The present inventors have confirmed that the difference of Ni weight percents between the Ni rich portions and the Ni poor portions discriminated by microscopic observation amounts to 2.0-2.5% in some cases. In view of the fact that the control of Ni weight percents in a forged or rolled product showing almost no segregation is made within a range not broader than 2%, it seems almost impossible to avoid local martensite transformation in a cast product at a room temperature or through a sub-zero cooling to slightly below the ice point.